The complete amino acid sequence of ferredoxin isolated from a green alga, Scenedesmus species, was determined by analyses of tryptic and chymotryptic digests, and cyanogen bromide cleavage of the S-carboxymethylcysteinylferredoxin. The total number of amino acid residues is 96, one less than for the higher plant ferredoxins. It is the first chloroplast-type ferredoxin found lacking in tryptophan and asparagine, and containing methionine. Evidently, neither tryptophan nor methionine can be essential for electron transfer activity. Six half-cystine residues were found in Scenedesmus ferredoxin as compared with only 5 in spinach and alfalfa ferredoxins. All of them were titratable with p-chloromercuribenzoate without prior reduction of the protein. Five are located in identical positions in all three ferredoxins. Similarity was observed between the ferredoxins of Scenedesmus, spinach, and alfalfa, with low values for minimum base differences per codon of 0.28 to 0.42. In the comparison of Scenedesmus and spinach ferredoxins there is a region which suggests a change produced by a frameshift mutation.